This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Longwall mining is by far the most efficient means of extracting coal in underground mines today. The longwall system consisting of the shearer, conveyor system, and hydraulic roof supports (shields) is constructed on one end of solid block of coal that ranges between 1000′ and 1,500′ wide, and 2 to 4 miles long. Once constructed the longwall system mines that entire block of coal taking cuts (passes) across the width of the panel (the face). As the coal is being mined across the face, the roof above must be temporarily supported to prevent the collapse of the material above. Once the shields advance to support newly exposed roof, they allow material that was once above them, to fall behind them.
The efficient advancement of these shield supports is where the present invention plays a key role. At times in Longwall mining, the immediate roof can deteriorate drastically between the time a cut is made, and the advancement of the shield supports. In some cases, the roof conditions can be so poor that the roof begins to fall immediately as the longwall mining machine cuts the face. When this occurs, a void is created above the shields that: 1) causes more material to fall onto the conveyor; usually in the form of large unmanageable pieces, and 2) prevents further advancement of the shields. The shield supports are connected to the conveyor system. Their progression consists of 1) collapsing to the point where they are no longer in contact with the roof, 2) advancing forward to support newly exposed roof, 3) repressurizing against the roof, and 4) pushing the conveyor system forward to position it for the longwall shearer's next pass along the face. If the shields extend fully but are unable to pressurize against the mine roof due to a void, the conveyor system cannot move forward and thus the longwall mining process is stopped. While the process is stalled with exposed rock above, gravity continues to act on the exposed rock causing more material to fall. The more rock that falls, the larger the existing void becomes. The larger the void becomes, the more rock falls, and so on. The amount of time that gravity is allowed to act on a particular area of unsupported top also serves to worsen the circumstances.